THEORIES OF SLEEP. 435 



Cerebral anemia may be merely a concomitant of sleep, but it has 

 frequently been held to be its immediate cause, the cells having pre- 

 viously been fatigued and suffered a lowering of functional capacity 

 which has made them increasingly susceptible to depressing influences. 

 This is the basis of Howell's theory. He has suggested that the 

 exhaustion of the vaso-motor center is what induces sleep. We know 

 that this center is in tonic activity, sending out impulses which hold 

 the blood-vessels in a state of constriction greater than is natural for 

 them. This tonic activity can only mean constant metabolism in the 

 cells of the center. Furthermore the center is subject to the play of 

 afferent impulses from all parts of the body. It is reflexly spurred 

 to action by every sensory impression through eye or ear. It is called 

 to respond in an appropriate manner to every change of posture or 

 other muscular movement. It does not escape the effects of psychic 

 processes, emotional states. Nothing is more natural than to suppose 

 that the nerve cells of the center become fatigued by this unceasing 

 activity. After the hours that we habitually number in a period of 

 waking it responds less and less readily to the demands made upon it. 

 It begins to lose its grip, so to speak, on the superficial and perhaps 

 the splanchnic vessels. The blood supply to the brain tends to become 

 less and the pressure in its arteries to be reduced. The subjective 

 consequence is drowsiness. If it is resisted by fixing the attention or 

 by exercise, the center rallies temporarily under the spurring, contract- 

 ing the vessels and turning the tide of blood back into the brain. But 

 the anemia soon returns, and the drowsiness becomes more compelling. 

 When the person lies down, a flood of sensory impulses that have been 

 pouring in from the contracting muscles is suddenly checked. The 

 eyes are closed and the stream of visual impulses ceases. With the 

 withdrawal of this reflex stimulation and the acquiescence of the will 

 the center relaxes still further its hold upon the cutaneous vessels, 

 the blood-flow in the brain becomes more reduced, and unconsciousness 

 comes on. During sleep the vaso-motor center is responsive to stimuli 

 from without as the plethysmographic experiments show. A sufficient 

 stimulus produces waking, and seems to operate by turning back into 

 the brain a sufficient quantity of blood displaced from the contracting 

 vessels of the skin. Such a stimulus must be a strong one in the flrst 

 hour or two of sleep, but later a much weaker one will answer. Sev- 

 eral physiologists have tested the depth of sleep at different hours of 

 the night, judging of it by the height from which a weight must be 

 dropped that the sound of its fall shall arouse the sleeper. All have 

 agreed that the greatest depth of sleep is reached as early as the second 

 hour. According to one writer it becomes steadily more shallow from 

 that time until the end. Others have observed a second, minor deep- 



